1. Technical Field
The present disclosure relates to an imaging device.
2. Description of the Related Art
There is a demand for an imaging device that can operate at high speed in performing distance measurement, ultra-high-speed imaging, or the like using fluorescence-lifetime imaging microscopy (FLIM) or time-of-flight method. For example, fluorescence-lifetime imaging microscopy involves applying a light pulse to a test sample and repeatedly detecting fluorescence emitted from the test sample at extremely short time intervals of several nanoseconds or so. It is expected that if the temporal resolution in measurement can be improved, new knowledge about the target to be measured will be obtained.
The temporal resolution in measurement using an imaging device depends on the operating speed of the pixels. For example, in detection using a complementary metal oxide semiconductor (CMOS) imaging device, a cycle consisting of the release of charge in photodiodes (the reset of the photodiodes), the accumulation of charge by exposure, and the transfer of charge to floating diffusion is repeated. That is, the temporal resolution in measurement depends on the time required by this cycle. In particular, the time required by the release of charge from the pixels and the transfer of charge to the floating diffusion in the cycle have a large effect on the high-speed operation of the imaging device.
K. Yasutomi, et. al., “A 0.3 mm-resolution Time-of-Flight CMOS range imager with column-gating clock-skew calibration,” ISSCC2014, Dig. pp. 132-133 proposes a structure in which discharge gates are disposed between photodiodes and drains for releasing charge. In this literature, pixels having such a structure are referred to as “draining-only modulation (DOM) pixels.” In the DOM pixels, the photodiodes release charge while the discharge gates are opened. On the other hand, the photodiodes can transfer charge to the floating diffusion while the discharge gates are closed. Since the time required to reset the photodiodes is made substantially zero, the DOM pixels have improved temporal resolution.